Modification of asphalt cement

ABSTRACT

This invention is based upon the discovery that a polydiene rubber that is comprised of repeat units that are derived from a conjugated diene monomer and sulfur can be used to improved the force ductility, elastic recovery, toughness and tenacity of asphalt cement. The polydiene rubber that is comprised of repeat units that are derived from a conjugated diene monomer and sulfur also exhibits excellent compatibility with asphalt. The repeat units in the polydiene rubber that are derived from sulfur are in the backbone of the polymer. These repeat units that are derived from sulfur typically contain from 2 to 8 sulfur atoms (-Sn-). The subject invention more specifically relates to a modified asphalt cement which is comprised of (i) from about 90 weight percent to about 99 weight percent asphalt; (ii) from about 1 weight percent to about 10 weight percent of a polydiene rubber that is comprised of repeat units that are derived from a conjugated diene monomer and sulfur. The-present invention also reveals an asphalt concrete which is comprised of (A), from about 90 weight percent to about 99 weight percent of an aggregate and (B) from about 1 weight percent to about 10 weight percent of a modified asphalt cement which is comprised of (i) from about 90 weight percent to about 99 weight percent asphalt; (ii) from about 1 weight percent to about 10 weight percent of a polydiene rubber that is comprised of repeat units that are derived from a conjugated diene monomer and sulfur. The present invention also discloses a process for preparing a modified asphalt cement which comprises (1) blending from about 1 to about 10 parts by weight of a polydiene rubber that is comprised of repeat units that are derived from a conjugated diene monomer and sulfur into from about 90 to about 99 parts by weight of asphalt at a temperature which is within the range of about 130° to about 230° C. to produce a polymer-asphalt blend; and (2) mixing from about 0.1 to about 3 parts by weight of sulfur into the polymer-asphalt blend to produce the modified asphalt cement.

This is a divisional of U.S. patent application Ser. No. 09/712,587,filed on Nov. 14, 2000, now U.S. Pat. No. 6,573,315 which claims thebenefit of U.S. Provisional Application Serial No. 60/193,223, filed onMar. 30, 2000.

BACKGROUND OF THE INVENTION

Since the days of the Roman Empire importance of a good transportationsystem that includes roads and highways has been appreciated. By about300 B.C., the first section of the Appian Way extending from Rome toCapua was built. Some of the more than 50,000 miles of roadwayultimately built in the Roman Empire was constructed with heavy stone.However, not much progress was made in the art of road construction fromthe era of the Roman Empire until the development of the motor vehicles,such as automobiles and trucks.

For centuries, stone blocks, wood blocks, vitrified brick and naturalasphalt (bitumen) have been used to pave roads and highways. However, atthe beginning of the automobile era, most rural roadway surfacingconsisted of broken stone or gravel. Such roads were often rough, dustyand clearly inadequate for modem automobile and truck traffic.

Today, the United States has the most extensive highway system in theworld with about 2,000,000 miles of paved road. Napoleon realized theimportance of roadway systems and built such a system in France whichtoday has the second most extensive system of paved roadways in theworld covering about 500,000 miles. Germany, Japan, Great Britain, Indiaand Australia also currently have systems of paved roads that extendwell over 100,000 miles. In addition to these public roadways, there arecountless paved driveways, parking lots, airport runways, and taxiwaysall over the world.

Today, roads, highways, driveways and parking lots are often paved withasphalt concrete. Pavement can be made with asphalt concrete that isdust-free, smooth and which offers the strength required for modemautomobile and heavy truck traffic. Asphalt concrete is generally madeby mixing aggregate (sand and gravel or crushed stone) with the properquantity of asphalt cement at an elevated temperature. The hot asphaltconcrete is then placed by a layering machine or paver on the surfacebeing paved and thoroughly rolled before the asphalt concrete mixturecools. The asphalt concrete is normally applied at a thickness varyingfrom about 25 to about 100 millimeters.

Asphalt concrete pavements can be made to be very smooth which offersoutstanding frictional resistance for vehicles operating thereon. Suchasphalt concrete pavement can also be repaired simply by addingadditional hot asphalt concrete to holes and other types of defectswhich develop in the surface. Asphalt concrete pavements can also beupgraded easily by adding additional layers of hot asphalt concrete toold surfaces which are in need of repair.

Even though asphalt concrete offers numerous benefits as a pavingmaterial, its use is not trouble-free. One major problem encounteredwith asphalt concrete pavements is the loss of the adhesive bond betweenthe aggregate surface and the asphalt cement. This breaking of theadhesive bond between the asphalt cement and the aggregate surface isknown as “stripping.” The stripping of asphalt binder from aggregatesurfaces results in shorter pavement life and many millions of dollarsof maintenance work on American highways each year. Reduction of thisstripping tendency is of great benefit for improving the condition ofroads and lowering road maintenance costs.

Over the years, various methods have been developed to reduce strippingtendencies. For instance, amines and lime are known to act asanti-stripping agents and are frequently applied to the surface of theaggregate prior to mixing it with the asphalt cement in making asphaltconcrete. U.S. Pat. No. 5,219,901 discloses a technique for reducingstripping tendencies which involves coating the aggregate with a thin,continuous film of a water-insoluble high molecular weight organicpolymer, such as an acrylic polymer or a styrene-acrylic polymer.

U.S. Pat. No. 5,262,240 discloses a technique for providing aggregatewith a high level of resistance to stripping by water, which comprises:(1) mixing the aggregate with latex to form a latex/aggregate mixturewhich is comprised of from about 0.005 weight percent to about 0.5weight percent dry polymer; (2) heating the latex/aggregate mixture to atemperature which is within the range of about 66° C. to about 232° C.;(3) maintaining the latex/aggregate mixture at said elevated temperaturefor a time which is sufficient to reduce the moisture content of thelatex/aggregate mixture below about 0.7 weight percent and to allow thepolymer in the latex to crosslink on the surface of the aggregate toproduce the coated aggregate.

At high service temperatures, such as those experienced on hot summerdays, asphalt concrete can experience rutting and shoving. On the otherhand, at low service temperatures, such as those experienced during coldwinter nights, asphalt concrete can also experience low temperaturecracking. To combat these problems, it is known in the art to modifyasphalt cements with rubbery polymers, such as styrene-butadiene rubber(SBR). In some approaches, the SBR is added to the asphalt as a dryrubber while in others it is added as a latex. Such modificationtechniques can greatly improve resistance to rutting, shoving and lowtemperature cracking. However, the rubbery polymers used in suchapplications have a tendency to phase separate from hot asphalt cementsdue to poor compatibility. A solution to the problem of poorcompatibility is offered by the technique disclosed in U.S. Pat. No.5,002,987.

U.S. Pat. No. 5,002,987 relates to a modified asphalt cement containingfrom about 90 to about 99 parts by dry weight of an asphalt cement andfrom about 1 to about 10 parts by dry weight of a rubber latex having aweight average molecular weight of less than 250,000 and a Mooneyviscosity of less than 50. The latex is a random polymer comprising fromabout 60 to 100 weight percent of at least one conjugated diolefincontaining from 4 to 6 carbon atoms and from about 0 to 40 weightpercent styrene. This latex polymer is highly compatible with theasphalt and provides good ductility that results in good resistance tolow temperature cracking. However, the utilization of the rubberypolymers described in U.S. Pat. No. 5,002,987 in asphalt cements providelittle improvement in elastic recovery or toughness. Thus, their useresults in compromised rutting and shoving characteristics. Thereaccordingly is a current need for a modifier which is compatible withasphalt cement and which improves the resistance of asphalt concretemade therewith to rutting, shoving and low temperature cracking.

U.S. Pat. No. 5,534,568 reveals an asphalt concrete which is comprisedof (A) from about 90 weight percent to about 99 weight percent of anaggregate and (B) from about 1 weight percent to about 10 weight percentof a modified asphalt cement which is comprised of (1) from about 90weight percent to about 99 weight percent of asphalt and (2) from about1 weight percent to about 10 weight percent of a rubbery polymer whichis comprised of repeat units which are derived from (a) about 64 weightpercent to about 84.9 weight percent of a conjugated diolefin monomer,(b) about 15 weight percent to about 33 weight percent of a vinylaromatic monomer and (c) about 0.1 weight percent to about 3 weightpercent of isobutoxymethyl acrylamide.

U.S. Pat. No. 4,145,322 discloses a process for making a bitumen-polymercomposition which consists of contacting with each other, at atemperature between 130° C. and 230° C., 80 to 98 weight percent of abitumen exhibiting a penetration value between 30 and 220, and 2 to 20weight percent of a block copolymer, with an average molecular weightbetween 30,000 and 330,000 having the theoretical formula S_(x)—B_(y) inwhich S corresponds to the styrene structure groups, B corresponds tothe conjugated diene structure groups, and x and y are integers,stirring the obtained mixture for at least two hours, then adding 0.1 to3 percent by weight of elemental sulfur with respect to the bitumen andmaintaining the mixture thus obtained under agitation for at least 20minutes.

Batch polymerization techniques are normally used in synthesizing blockcopolymers that are utilized in modifying asphalt in order to attaindesired performance characteristics. However, it is highly desirablefrom a cost standpoint to synthesize such polymers by utilizingcontinuous polymerization techniques. It would also be highly desirableto increase the force ductility, elastic recovery, toughness andtenacity of asphalt which is modified with such polymers. U.S. Pat. No.5,986,010 discloses a technique for synthesizing, by a continuouspolymerization process, a styrene-butadiene polymer which is highlysuitable for modifying asphalt to improve force ductility, elasticrecovery, toughness and tenacity.

U.S. Pat. No. 5,986,010 specifically discloses a process forsynthesizing a styrene-butadiene polymer which is particularly usefulfor modifying asphalt by a continuous polymerization process, saidprocess comprising the steps of: (1) continuously charging 1,3-butadienemonomer, an organo lithium compound, a polar modifier and an organicsolvent into a first polymerization zone, (2) allowing the 1,3-butadienemonomer to polymerize in said first polymerization zone to a conversionof at least about 90 percent to produce a living polymer solution whichis comprised of said organic solvent and living polybutadiene chainshaving a number average molecular weight which is within the range ofabout 20,000 to about 60,000, (3) continuously withdrawing said livingpolymer solution from said first reaction zone, (4) continuouslycharging styrene monomer, divinyl benzene and the living polymersolution being withdrawn from the first polymerization zone into asecond polymerization zone, (5) allowing the styrene monomer and divinylbenzene monomer to polymerize in said second polymerization zone toproduce a solution of styrene-butadiene polymer having a number averagemolecular weight which is within the range of about 30,000 to about85,000 and (6) continuously withdrawing the solution of saidstyrene-butadiene polymer from the second polymerization zone.

U.S. Pat. No. 6,136,899 discloses an asphalt concrete which is comprisedof (A) from about 90 weight percent to about 99 weight percent of anaggregate and (B) from about 1 weight percent to about 10 weight percentof a modified asphalt cement which is comprised of (1) from about 90weight percent to about 99 weight percent of asphalt; and (2) from about1 weight percent to about 10 weight percent of a rubbery polymer whichis comprised of repeat units which are derived from styrene and1,3-butadiene, wherein the styrene-butadiene rubber composition has anumber average molecular weight as determined by field flowfractionation which is within the range of about 50,000 to 150,000 andwherein the styrene-butadiene rubber has a light scattering torefractive index ratio which is within the range of 1.8 to 3.9. Therubbery polymer disclosed in U.S. Pat. No. 6,136,899 is comprised of ablend of (i) a high molecular weight styrene-butadiene rubber having aweight average molecular weight of at least about 300,000 and (ii) a lowmolecular weight styrene-butadiene rubber having a weight averagemolecular weight of less than about 280,000; wherein the ratio of thehigh molecular weight styrene-butadiene rubber to the low molecularweight styrene-butadiene rubber is within the range of about 80:20 toabout 25:75; and wherein the bound styrene content of the high molecularweight styrene-butadiene rubber differs from the bound styrene contentof the low molecular weight styrene-butadiene rubber by at least 5percentage points.

SUMMARY OF THE INVENTION

This invention is based upon the discovery that polydiene rubber that iscomprised of repeat units that are derived from a conjugated dienemonomer and sulfur can be used to improved the force ductility, elasticrecovery, toughness and tenacity of asphalt cement. Surprisingly, thepolydiene rubber that is comprised of repeat units that are derived froma conjugated diene monomer and sulfur exhibits excellent compatibilitywith asphalt.

The subject invention more specifically relates to a modified asphaltcement which is comprised of (i) from about 90 weight percent to about99 weight percent asphalt; (ii) from about 1 weight percent to about 10weight percent of a polydiene rubber that is comprised of repeat unitsthat are derived from a conjugated diene monomer and sulfur.

The present invention also discloses an asphalt concrete which iscomprised of (A) from about 90 weight percent to about 99 weight percentof an aggregate and (B) from about 1 weight percent to about 10 weightpercent of a modified asphalt cement which is comprised of (i) fromabout 90 weight percent to about 99 weight percent asphalt; (ii) fromabout 1 weight percent to about 10 weight percent of a polydiene rubberthat is comprised of repeat units that are derived from a conjugateddiene monomer and sulfur.

The subject invention further reveals a process for preparing a modifiedasphalt cement which comprises (1) blending from about 1 to about 10parts by weight of a polydiene rubber that is comprised of repeat unitsthat are derived from a conjugated diene monomer and sulfur into fromabout 90 to about 99 parts by weight of asphalt at a temperature whichis within the range of about 130° to about 230° C. to produce apolymer-asphalt blend; and (2) mixing from about 0.1 to about 3 parts byweight of sulfur into the polymer-asphalt blend to produce the modifiedasphalt cement.

DETAILED DESCRIPTION OF THE INVENTION

The polydiene rubber that is comprised of repeat units that are derivedfrom a conjugated diene monomer and sulfur that is used to modifyasphalt cement in accordance with this invention can be made by a batchor a continuous emulsion polymerization using a free radical initiatorsystem. This is carried out by adding the conjugated diolefin monomer,sulfur, water, a free radical generator, and a soap system to apolymerization zone to form an aqueous polymerization medium. Thepolymerization zone will normally be a reactor or series of two or morereactors. Polymerization is initiated with the free radical generator.This polymerization reaction results in the formation of a latex of thepolydiene rubber that is comprised of repeat units that are derived fromthe conjugated diene monomer and sulfur.

The conjugated diolefin monomer will generally contain from 4 to 12carbon atoms. Those containing from 4 to 8 carbon atoms are generallypreferred for commercial purposes. For similar reasons, 1,3-butadieneand isoprene are the most commonly utilized conjugated diolefinmonomers. Some additional conjugated diolefin monomers that can beutilized include 2,3-dimethyl-1,3-butadiene, piperylene,3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the like, alone or inadmixture.

Other ethylenically unsaturated monomers can also be copolymerized intothe polydiene rubber that is comprised of repeat units that are derivedfrom the conjugated diene monomer and sulfur. Some representativeexamples of additional ethylenically unsaturated monomers that canpotentially be synthesized into the polydiene rubber include alkylacrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate,methyl methacrylate, and the like; vinylidene monomers having one ormore terminal CH₂═CH-groups; vinyl aromatics, such as styrene,α-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene, and thelike; vinyl halides, such as 1,1-dichloroethene (vinylidene chloride),1,2-dichloroethene, and the like; α,β-olefinically unsaturated nitriles,such as acrylonitrile and methacrylonitrile; α,β-olefinicallyunsaturated amides, such as acrylamide, N-methyl acrylamide,N,N-dimethylacrylamide, methacrylamide, and the like.

The polydiene rubber that is comprised of repeat units that are derivedfrom a conjugated diene monomer and sulfur can be a copolymer of one ormore conjugated diene monomers with one or more other ethylenicallyunsaturated monomers. Such polydiene rubbers will normally contain fromabout 50 weight percent to about 99 weight percent conjugated diolefinmonomers and from about 1 weight percent to about 50 weight percent ofthe other ethylenically unsaturated monomers in addition to theconjugated diolefin monomers. For example, copolymers of conjugateddiolefin monomers with vinylaromatic monomers, such as styrene-butadienerubbers which contain from 50 to 95 weight percent conjugated diolefinmonomers and from 5 to 50 weight percent vinylaromatic monomers, areuseful in the asphalt compositions of this invention. In such cases, thepolydiene rubber will, of course, also contain repeat units that arederived from sulfur.

Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers that can be incorporated into thepolydiene rubbers employed in the asphalt compositions of thisinvention. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer that is known topolymerize with free radical initiators can be used. Such vinyl aromaticmonomers typically contain from 8 to 20 carbon atoms. Usually, the vinylaromatic monomer will contain from 8 to 14 carbon atoms. The most widelyused vinyl aromatic monomer is styrene. Some examples of vinyl aromaticmonomers that can be utilized include styrene, 1-vinylnaphthalene,2-vinylnaphthalene, α-methylstyrene, 4-phenylstyrene, 3-methylstyrene,and the like. Terpolymer rubbers of 1,3-butadiene, styrene, and sulfurare particularly preferred.

In synthesizing sulfur containing SBR latex generally from about 10weight percent to about 40 weight percent styrene and from about 60weight percent to about 90 weight percent 1,3-butadiene arecopolymerized. It is typically preferred for the SBR to contain fromabout 20 weight percent to about 30 weight percent styrene and fromabout 70 weight percent to about 80 weight percent 1,3-butadiene. It isnormally most preferred for the SBR to contain from about 24 weightpercent to about 28 weight percent styrene and from about 72 weightpercent to about 76 weight percent 1,3-butadiene. Like ratios of styrenemonomer and butadiene monomer will accordingly be charged into thepolymerization zone.

U.S. Pat. No. 6,136,899 discloses a styrene-butadiene rubber (SBR)composition that can be used to modify asphalt cement. Thestyrene-butadiene rubber composition disclosed by U.S. Pat. No.6,136,899 is a blend of (i) a high molecular weight styrene-butadienerubber having a weight average molecular weight of at least about300,000 and (ii) a low molecular weight styrene-butadiene rubber havinga weight average molecular weight of less than about 280,000; whereinthe ratio of the high molecular weight styrene-butadiene rubber to thelow molecular weight styrene-butadiene rubber is within the range ofabout 80:20 to about 25:75; and wherein the bound styrene content of thehigh molecular weight styrene-butadiene rubber differs from the boundstyrene content of the low molecular weight styrene-butadiene rubber byat least 5 percentage points. These styrene-butadiene rubbercompositions are comprised of repeat units which are derived fromstyrene and 1,3-butadiene, wherein the styrene-butadiene rubbercomposition has a number average molecular weight as determined by fieldflow fractionation which is within the range of about 50,000 to 150,000and wherein the styrene-butadiene rubber has a light scattering torefractive index ratio which is within the range of 1.8 to 3.9. Thestyrene-butadiene rubber in the compositions disclosed by U.S. Pat. No.6,136,899 can be modified by conducting the emulsion polymerization inthe presence of sulfur in accordance with the technique of thisinvention. The styrene-butadiene rubber compositions made by such atechnique are comprised of repeat units which are derived from styrene,1,3-butadiene, and sulfur, wherein the styrene-butadiene rubbercomposition has a number average molecular weight as determined by fieldflow fractionation which is within the range of about 50,000 to 150,000and wherein the styrene-butadiene rubber has a light scattering torefractive index ratio which is within the range of 1.8 to 3.9. Suchstyrene-butadiene rubber compositions can be used advantageously inmodifying asphalt compositions. The teachings of U.S. Pat. No. 6,136,899are accordingly incorporated herein by reference in their entirety.

The amount of sulfur charged into the polymerization zone will typicallybe within the range of about 0.01 phm (parts per 100 parts by weight ofmonomer) to about 20 phm and will more typically be within the range ofabout 0.05 phr to about 5 phm. The amount of sulfur charged into thepolymerization zone will preferably be within the range of about 0.1 phmto about 1 phm and will more preferably be within the range of about0.03 phr to about 0.7 phm. The sulfur charged into the polymerizationzone is conventional elemental sulfur. The sulfur will typically be inthe form of a powder. It is preferred for the sulfur to be in the formof a powder having a small particle size. For instance, “rubber makerssulfur” can be used utilized. Optimally, a finely divided dispersion ofsulfur in water can be used.

Essentially any type of free radical generator can be used to initiatethe free radical emulsion polymerization. For example, free radicalgenerating chemical compounds, ultra-violet light or radiation can beused. In order to ensure a satisfactory polymerization rate, uniformity,and a controllable polymerization, free radical generating chemicalagents which are water or oil soluble under the polymerizationconditions are typically used.

Some representative examples of free radical initiators which arecommonly used include the various peroxygen compounds such as potassiumpersulfate, ammonium persulfate, benzoyl peroxide, hydrogen peroxide,di-t-butyl peroxide, dicumyl peroxide, 2,4-dichlorobenzoyl peroxide,decanoyl peroxide, lauryl peroxide, cumene hydroperoxide, p-menthanehydroperoxide, t-butyl hydroperoxide, acetyt acetone peroxide, dicetylperoxydicarbonate, t-butyl peroxyacetate, t-butyl peroxymaleic acid,t-butyl peroxybenzoate, acetyl cyclohexyl sulfonyl peroxide, and thelike; the various azo compounds such as 2-t-butylazo-2-cyanopropane,dimethyl azodiisobutyrate, azodiisobutyronitrile,2-t-butylazo-1-cyanocyclohexane, 1-t-amylazo-1-cyanocyclohexane, and thelike; the various alkyl perketals, such as2,2-bis-(t-butylperoxy)butane, ethyl 3,3-bis(t-butylperoxy)butyrate,1,1-di-(t-butylperoxy) cyclohexane, and the like. Persulfate initiators,such as potassium persulfate and ammonium persulfate are especiallyuseful in such aqueous emulsion polymerizations.

The amount of initiator employed will vary with the desired molecularweight of the SBR being synthesized. Higher molecular weights areachieved by utilizing smaller quantities of the initiator and lowermolecular weights are attained by employing larger quantities of theinitiator. However, as a general rule from 0.005 to 1 phm (parts byweight per 100 parts by weight of monomer) of the initiator will beincluded in the reaction mixture. In the case of metal persulfateinitiators most commonly from 0.1 to 0.5 phm will be employed in thepolymerization medium.

A wide variety of soap systems can be used to emulsify thepolymerization medium. For instance, an anionic, cationic or non-ionicemulsifier can be employed. A combination of rosin acid and fatty acidemulsifiers can be employed with excellent results. In such systems, theweight ratio of fatty acid soaps to rosin acid soaps will be within therange of about 50:50 to 90:10. It is normally preferred for the weightratio of fatty acid soaps to rosin acid soaps to be within the range of60:40 to 85:15. It is normally more preferred for the weight ratio offatty acid soaps to rosin acid soaps to be within the range of 75:25 to82:18. All of the soap is charged into the first polymerization zone inpracticing this invention. The total amount of soap employed willnormally be within the range of about 1 phm to 5 phm. It is typicallypreferred to utilize a level of soap that is within the range of about 2phm to about 3.5 phm. In most cases it will be most preferred to use anamount of the soap system which is within the range of about 2.5 phm to3 phm. The precise amount of the soap system required in order to attainoptimal results will, of course, vary with the specific soap systembeing used. However, persons skilled in the art will be able to easilyascertain the specific amount of soap system required in order to attainoptimal results.

The free radical emulsion polymerization will typically be conducted ata temperature which is within the range of about 20° F. to about 80° F.It is generally preferred for the polymerization to be carried out at atemperature that is within the range of 30° F. to about 65° F. It istypically more preferred to utilize a polymerization temperature whichis within the range of about 45° F. to about 55° F. To increaseconversion levels, it can be advantageous to increase the temperature asthe polymerization proceeds.

The polymerizations employed in making the polydiene rubber that iscomprised of repeat units that are derived from a conjugated dienemonomer and sulfur are typically initiated by adding the initiator tothe aqueous polymerization medium that contains the conjugated dienemonomer, sulfur, water and emulsifier. Such polymerizations aretypically carried out utilizing continuous polymerization techniques. Insuch continuous polymerizations, monomer and initiator are continuouslyadded to the organic polymerization medium with a latex of the polydienerubber that is comprised of repeat units that are derived from aconjugated diene monomer and sulfur being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem.

The polydiene rubber that is comprised of repeat units that are derivedfrom a conjugated diene monomer and sulfur. The polydiene rubber can, ofcourse, be derived from more than one conjugated diolefin monomers andone or more additional monomers, such as vinyl aromatic monomers. Therepeat units in the polydiene rubber that are derived from sulfur are inthe backbone of the polymer. These repeat units that are derived fromsulfur typically contain from 2 to 8 sulfur atoms (—S_(n)—). It isbelieved that S₈ molecules are incorporated into the backbone of thepolymer by a ring opening reaction. In any case, the latex of thepolydiene rubber is comprised of water, an emulsifier, and the polydienerubber.

Asphalt cement can be modified with the polydiene rubber that iscomprised of repeat units that are derived from a conjugated dienemonomer and sulfur by simply mixing a latex of the polydiene rubber intohot asphalt. The latex of the polydiene rubber will typically be mixedinto the asphalt at a temperature which is within the range of about130° C. to about 230° C. The latex of the polydiene rubber may be addedto the asphalt in an amount which is within the range of from about 1part by weight to about 10 parts by weight (based upon the dry weight ofthe rubber). Preferably, from about I part by weight to about 6 parts byweight of the polydiene rubber is added with amounts within the range offrom about 2 parts by weight to about 4 parts by weight beingparticularly preferred. After the latex of the polydiene rubber has beenwell dispersed throughout the asphalt, elemental sulfur can optionallybe added to the polymer/asphalt blend. Normally from about 0.1 to about5 parts by weight of sulfur is added per 100 parts by weight of thepolydiene, rubber. In most cases, it is preferred to utilize from about1 to about 4 parts by weight of sulfur per 100 parts by weight of thepolydiene rubber. It is typically more preferred to utilize from about 2to about 3 parts by weight of sulfur per 100 parts by weight of thepolydiene rubber. After the polydiene rubber and the sulfur has beenthoroughly mixed with the asphalt cement, one should store the modifiedasphalt cement at elevated temperatures to avoid solidification prior touse.

Virtually any type of asphalt can be employed in making the asphaltcement compositions of this invention. Asphalt is defined by ASTM as adark brown to black cementitious material in which the predominantconstituents are bitumens that occur in nature or are obtained inpetroleum processing. Asphalts characteristically contain very highmolecular weight hydrocarbons called asphaltenes. These are essentiallysoluble in carbon disulfide and aromatic and chlorinated hydrocarbons.Bitumen is a generic term defined by ASTM as a class of black ordark-colored (solid, semi-solid or viscous) cementitious substances,natural or manufactured, composed principally of high molecular weighthydrocarbons, of which asphalts, tars, pitches, asphaltites are typical.ASTM further classifies asphalts or bituminous materials as solids,semi-solids or liquids using a penetration test for consistency orviscosity. In this classification, solid materials are those having apenetration at 25° C. under a load of 100 grams applied for 5 seconds,of not more than 10 decimillimeters (1 millimeter). Semi-solids arethose having a penetration at 25° C. under a load of 100 grams appliedfor 5 seconds of more than 10 decimillimeters (1 millimeter) and apenetration at 25° C. under a load of 50 grams applied for 1 second ofnot more than 35 millimeters. Semi-solid and liquid asphalts predominatein commercial practice today.

Asphalts are usually specified in several grades for the same industry,differing in hardness and viscosity. Specifications of paving asphaltcements generally include five grades differing in either viscositylevel at 60° C. or penetration level. Susceptibility of viscosity totemperatures is usually controlled in asphalt cement by its viscositylimits at a higher temperature such as 135° C. and a penetration orviscosity limit at a lower temperature such as 25° C. For asphaltcements, the newer viscosity grade designation is the mid-point of theviscosity range.

The asphalt materials that may be used in the present invention arethose typically used for road paving, repair and maintenance purposes.Petroleum asphalts are the most common source of asphalt cements.Petroleum asphalts are produced from the refining of petroleum and usedpredominantly in paving and roofing applications. Petroleum asphalts,compared to native asphalts, are organic with only trace amounts ofinorganic materials. Some representative examples of asphalt cementsthat may be used in the present invention have an ASTM grade of AC-2.5,AC-5, AC-10, AC-20 and AC-40. The preferred asphalt cements includeAC-5, AC-10 and AC-20.

The present invention may contain other conventional additives inaddition to the polydiene rubber, the asphalt cement, and optionally,the sulfur. Examples of conventional additives include antistrippingcompounds, fibers, release agents and fillers. Some specific examples ofadditives that can be employed include kaolin clay, calcium carbonate,bentonite clay, and cellulose fibers.

After the asphalt cement has been modified, it can be mixed withaggregate to make asphalt concrete using standard equipment andprocedures utilized in making asphalt concrete. As a general rule, fromabout 1 weight percent to about 10 weight percent of the modifiedasphalt cement and from about 90 weight percent to about 99 weightpercent aggregate will be included in the asphalt concrete. It is moretypical for the asphalt concrete to contain from about 3 weight percentto about 8 weight percent of the modified asphalt cement and from about92 weight percent to about 97 weight percent of the aggregate. It isnormally preferred for the asphalt concrete to contain from about 4weight percent to about 7 weight percent of the modified asphalt cementand from about 93 weight percent to about 96 weight percent of theaggregate.

The aggregate is mixed with the asphalt to attain an essentiallyhomogeneous asphalt concrete. The aggregate is mixed with the asphaltcement utilizing conventional techniques and standard equipment. Forinstance, the aggregate can be mixed with asphalt to produce asphaltconcrete on a continuous basis in a standard mixer.

Standard aggregate can be utilized in the practice of this invention.The aggregate is essentially a mixture containing rocks, stones, crushedstone, gravel and/or sand. The aggregate will typically have a widedistribution of particle sizes ranging from dust to golf ball size. Thebest particle size distribution varies from application to application.In many cases, it will be advantageous to coat the aggregate with latexin accordance with the teachings of U.S. Pat. No. 5,262,240 to improveresistance to stripping by water.

The asphalt concrete made using the modified asphalt cement of thisinvention can then be used to pave roads, highways, exit ramps, streets,driveways, parking lots, airport runways or airport taxiways utilizingconventional procedures. However, pavements made utilizing the asphaltconcretes of this invention offer resistance to rutting, shoving and lowtemperature cracking. Additionally, they can be applied withoutencountering processing difficulties due to the latex used for themodification being incompatible with the asphalt.

This invention is illustrated by the following examples that are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

EXAMPLE 1

In this experiment a latex of a polydiene rubber containing repeat unitsthat were derived from 1,3-butadiene, styrene, and sulfur was prepared.In the procedure used emulsion rubbers containing several levels ofsulfur added were prepared in 750 ml polymerization bottles. The recipetypically used is shown in Table I.

TABLE I A B Act Act Actual Weight Act Actual Weight Material Code (%)Pts (grams) Pts (grams) A. RO Water w 100 156.25 250.00 Potassiumphosphate n 100 0.178 0.28 Hydrogenated mixed fatty acid n 10 2.661 4.26Potassium soap of n 20 1.395 2.23 disproportionated tall oil rosinSodium naphthalenesulfonate n 47.5 0.192 0.31 formaldehyde SiponateA246L sodium lauryl n 40 0.017 0.03 sulfonate B. Sulfur n 100 0 0.5 0.80RO Water w 100 38.75 62.00 100 62.00 C. Styrene m 100 25 40.00 Tertarymercaptan n 100 0.1 0.16 D. Sodium hydrosulfite n 100 0.1 0.16 RO Waterw 100 1 1.60 E. RO Water w 100 4 6.40 Sodium formaldehyde sulfoxylate n76.62 0.03 0.05 Ethylenediaminetetraacetic acid n 87.17 0.01 0.02 F.Butadiene b 100 75 120.00 G. Pinane hydroperoxide n 44 0.04 0.06 H. ROWater w 100 3.1 4.96 Sodium dimethyldithio carbamate n 40 0.1 0.16Diethyl hydroxylamine n 85 0.01 0.02

The materials in Section A were masterbatched together, then chargedinto 750 ml bottles. The pH was typically within the range of 10.5 to11.0, if not it was adjusted with the addition of a small amount of 10%sodium hydroxide solution. The amount of sulfur charged (Section B)ranged from 0 to 2.0 parts. The sulfur was weighed into a small bottle,RO water was added and then the mixture was sparged with nitrogen for 10minutes before being added to the polymerization bottle. The materialsof Sections C, D, E and F were added, the bottle was capped, and thenplaced into a water bath that was maintained at a temperature of 50° F.(10° C.). The bottles were tumbled for about 15 minutes to reachconstant temperature, then the pinane hydroperoxide of Section G wasadded via syringe. The bottles were tumbled and maintained at atemperature of 10° C. until a monomer conversion of 65% (as determinedby solids sampling) was attained. Polymerization times ranged from 6 to12 hours. Additional shots of activator (see Section E) and pinanehydroperoxide (see Section G) were added as needed to speed up thereaction if the solids content was low after two hours of polymerizationtime. The polymerization was shortstopped with a conventionaldimethyldithiocarbamate/diethyl hydroxylamine solution in water (seeSection H).

Latex from the polymerization bottles were stripped on a Buchi Rotoevaporator to remove unreacted styrene monomer. Typically, 2000 grams ofwater was added, then stripped off. This reduced the residual styrenelevel to an undetectable level. The latex was then filtered throughcheesecloth.

EXAMPLE 2

In this experiment, asphalt compositions were made with the latex of themodified sulfur containing polydiene rubber synthesized in Example 1. Inone case, 2 weight percent sulfur was added to the asphalt compositionand compared to an asphalt composition where sulfur was not added. AAC-20 asphalt from Kansas was used in this experiment. In the procedureused, 15.5 grams of the rubber was slowly stirred into the asphalt overa period of about 45 minutes at a temperature of about 350° F.-360° F.(177° C.-182° C.). The rubbers were added at a level of 3 weightpercent, based upon the total weight of the polymer/asphalt blends.Then, the polymer/asphalt blends were mixed for about 15 minutes in aRoss high speed mixer which was operated at a speed of 4200 rpm.Elemental sulfur was subsequently mixed into the polymer/asphalt blendover a period of about 2 minutes (in making one composition) and themixture was then slowly stirred over a period of 1 hour at a temperatureof 350° F.-360° F. (177° C.-182° C.).

The physical properties of the modified asphalt cements made were thendetermined using standard test procedures. The strength and flexibilityof the asphalt binder cement at moderate or low temperatures aremeasured by force ductility and tenacity. These properties measure theresistance to deformation. Increasing strength gives greater resistanceto surface abrasion and wear and provides better retention of aggregate.Ductility was determined by ASTM D113. The force ductility, elasticrecovery, and tenacity of the two modified asphalt samples is reportedin Table II.

TABLE II No Sulfur Added Sulfur Added Ductility, 4° C.; cm 23.5  30Elastic Recovery @ 10° C. 40.0% 52.5% Force Ductility, 4° C., 800% 2.3lbs 3.2 lbs Tenacity, in-lbs 26.4 110.5 Compatibility (° C.)  0.6  0.5

The polydiene rubber containing repeat units that were derived fromsulfur was also determined to have excellent compatibility with asphalt.This was determined by utilizing a separation test wherein the modifiedasphalt sample was placed in a tube having a diameter of 1 inch (2.54cm) and a length of 5.5 inches (14 cm) and heat in an oven at 325° F.(163° C.) for 48 hours. The tube was maintained in a vertical positionthroughout the heating step. The tube containing the asphalt sample wasthen placed in a freezer at about 20° F. (−7° C.) for a minimum of 4hours. Then the sample was removed from the freezer and cut into threeportions of equal length. The ring and ball softening point of the topand bottom portions of the sample was then determined by ASTM MethodD36. Compatibility is considered to be excellent in cases where thedifference in temperature between the softening points between the topand bottom samples is no greater than 2° C. In the case at hand, thistemperature difference was only 0.5° C. in one case and 0.6° C. in theother case. Thus, the polydiene rubbers exhibited excellentcompatibility with the asphalt. As can be seen from Table II, theaddition of sulfur the asphalt composition greatly increased thetenacity.

EXAMPLE 3

In this experiment a conventional styrene-butadiene rubber that did notcontain repeat units derived from sulfur (made with a recipe that wasotherwise identical) was used to modify asphalt using the proceduredescribed in Example 2. However, the rubber was grossly incompatiblewith the asphalt and physical properties were not tested.

EXAMPLE 4

The procedure described in Example 2 was repeated in this experimentexcept that AC-20 asphalt from Mississippi was substituted for the AC-20asphalt from Kansas used in Example 2. The ductility, elastic recovery,tenacity, and compatibility of the two modified asphalt samples isreported in Table III.

TABLE III No Sulfur Added Sulfur Added Ductility, 4° C.; cm 80 — ElasticRecovery @ 10° C. 40.0% 55.0% Force Ductility, 4° C., 800% 0.9 2.0Tenacity, in-lbs 44.9 107.0 Compatibility (° C.) 0.1 0.3

As can be seen from Table III, the polydiene rubber had excellentcompatibility with the AC-20 asphalt from Mississippi. Again tenacitywas greatly increased by adding free sulfur to the asphalt composition.

EXAMPLE 5

The procedure described in Example 2 was repeated in this experimentexcept that AC-20 asphalt from Texas and AC-20 asphalt (from Kentucky)was substituted for the AC-20 asphalt from Kansas used in Example 2. Inthe procedure used 3 weight percent free sulfur was added in both cases.The ductility, elastic recovery, tenacity, and compatibility of the twomodified asphalt samples is reported in Table IV.

TABLE IV TFA Asphalt Asphalt Ductility, 4° C.; cm 48 25 Elastic Recovery@ 10° C. 45.0% 35.0% Force Ductility, 4° C., 800% 1.0 1.5 Tenacity,in-lbs 49.1 39.8 Compatibility (° C.) 0.5 0.9

As can be seen from Table IV, the polydiene rubber had excellentcompatibility with both the AC-20 asphalt from Texas and the AC-20asphalt from Kentucky. It is unusual for a polymer to have goodcompatibility with such a wide array of asphalts. However, byincorporating sulfur into the backbone of the rubber excellentcompatibility with all of the asphalt samples which came from a widevariety of geographic locations was realized.

EXAMPLE 6

In this experiment a conventional styrene-butadiene rubber (that did notcontain repeat units derived from sulfur) was used to modify AC-20asphalt from Texas and AC-20 asphalt from Kentucky using the proceduredescribed in Example 2. However, the rubber was grossly incompatiblewith both of the asphalt samples and physical properties were nottested.

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

What is claimed is:
 1. A process for preparing a modified asphalt cementwhich comprises (1) blending from about 1 to about 10 parts by weight ofa polydiene rubber that is comprised of repeat units that are derivedfrom a conjugated diene monomer and sulfur into from about 90 to about99 parts by weight of asphalt at a temperature which is within the rangeof about 130° to about 230° C. to produce a polymer-asphalt blend; and(2) mixing from about 0.1 to about 5 parts by weight of sulfur into thepolymer-asphalt blend to produce the modified asphalt cement.
 2. Aprocess as specified in claim 1 wherein from about 1 to about 4 parts byweight of sulfur is mixed into the polymer-asphalt blend to produce themodified asphalt cement.
 3. A process as specified in claim 1 whereinfrom about 2 to about 3 parts by weight of sulfur is mixed into thepolymer-asphalt blend to produce the modified asphalt cement.
 4. Aprocess as specified in claim 1 wherein the repeat units in thepolydiene rubber that are derived from sulfur are in the backbone of thepolydiene rubber.
 5. A process as specified in claim 4 wherein therepeat units that are derived from sulfur contain from 2 to 8 sulfuratoms.
 6. A process as specified in claim 1 wherein the polydiene rubberis synthesized by the free radical polymerization of the conjugateddiolefin monomer and sulfur in an aqueous polymerization medium.
 7. Aprocess as specified in claim 6 wherein the sulfur in present in theaqueous polymerization medium at a level which is within the range ofabout 0.01 phm to about 20 phm.
 8. A process as specified in claim 6wherein the sulfur in present in the aqueous polymerization medium at alevel which is within the range of about 0.05 phm to about 5 phm.
 9. Aprocess as specified in claim 6 wherein the sulfur in present in theaqueous polymerization medium at a level which is within the range ofabout 0.1 phm to about 1 phm.
 10. A process as specified in claim 6wherein the sulfur in present in the aqueous polymerization medium at alevel which is within the range of about 0.3 phm to about 0.7 phm.
 11. Aprocess as specified in claim 6 wherein the polymerization is conductedat a temperature which is within the range of about 20° C. to about 80°C.
 12. A process as specified in claim 7 wherein the polymerization isconducted at a temperature which is within the range of about 30° C. toabout 65° C.
 13. A process as specified in claim 8 wherein thepolymerization is conducted at a temperature which is within the rangeof about 45° C. to about 55° C.
 14. A process as specified in claim 1wherein the conjugated diolefin monomer is 1,3-butadiene.
 15. A processas specified in claim 14 wherein the polydiene rubber is furthercomprised of repeat units that are derived from a vinyl aromaticmonomer.
 16. A process as specified in claim 15 wherein the vinylaromatic monomer is styrene.
 17. A process as specified in claim 1wherein the polydiene rubber is utilized at a level which is within therange of about 1 weight percent to about 6 weight percent, and whereinthe asphalt is utilized at a level which is within the range of about 94weight percent to about 99 weight percent.
 18. A process as specified inclaim 1 wherein the polydiene rubber is utilized at a level which iswithin the range of about 2 weight percent to about 4 weight percent,and wherein the asphalt is utilized at a level which is within the rangeof about 96 weight percent to about 98 weight percent.
 19. A process asspecified in claim 1 wherein the asphalt has an ASTM grade of AC-10. 20.A process as specified in claim 3 wherein the asphalt has an ASTM gradeof AC-20.